1. Field of the Invention
This invention relates to a light emitting device and a method of manufacturing a light emitting device, and more particularly to a light emitting device and a method of manufacturing a light emitting device where current injection from the electrode and light extraction efficiency can be improved.
2. Background Art
Typically, a light emitting diode has the following basic structure: an active layer is sandwiched between a p-type cladding layer and an n-type cladding layer to form a double heterostructure, contact layers are provided above and below the double heterostructure, and a p-side electrode and an n-side electrode are connected to the contact layers, respectively. The active layer, the n-type cladding layer, and the p-type cladding layer are illustratively made of mixed crystals of group III-V compound semiconductors such as InGaAlP, InAlP, and InGaP. A prescribed emission color within the wavelength range from green to red can be obtained by adjusting the elemental composition ratio of the active layer.
A light emitting diode emits light as a result of recombination of holes and electrons in the active layer, the holes and electrons being injected from the p-side electrode and the n-side electrode, respectively. Thus, in order to achieve higher emission efficiency, it is important to improve the efficiency of injecting holes and electrons from both electrodes into the semiconductor layers. The contact layers serve to facilitate injecting holes or electrons from the electrodes. Typically, the p-type contact layer is made of p-type GaAs, and the n-type contact layer is made of n-type GaAs. Likewise, the p-side electrode and the n-side electrode formed on the contact layers should efficiently inject holes or electrons into the corresponding contact layers, and thus require ohmic contact with the contact layers. Typically, these electrodes are metal films made of AuGe alloy or AuZn alloy, for example.
However, in the contact layers and electrodes on the light extraction side, it is preferable to exclude elements that absorb or block light in order to increase the light extraction efficiency. The thickness of the GaAs layer serving as a contact layer is preferably about 100 nanometers (nm) or less so as to reduce light absorption and avoid degradation of electric conductivity. On the other hand, with respect to electrodes, because metal films cannot transmit light, it is preferable to minimize the area of the electrode. However, a certain amount of area is needed in view of the efficiency of transport and injection of electrons or holes to the entire surface of the contact layer.
As described above, it is important to improve the electrode on the light extraction side for improving the light extraction efficiency and achieving a light emitting diode with high emission efficiency. To this end, electrodes on the light extraction side made of transparent electrodes such as indium tin oxides (ITO) are proposed (JP2003-174197A).
However, when an ITO electrode forms a junction with the GaAs contact layer, it is difficult to obtain ohmic contact with sufficiently low contact resistance.